Mass spectrometry will be applied to structural and analytical problems involving naturally modified nucleosides. Structures of new nucleosides from transfer RNA, ribosomal RNA and small nuclear RNA will be studied, principally using high resolution mass spectrometry. Distribution of the hypermodified base queuine in prokaryotes and eukaryotes will be measured by gas chromatography-mass spectrometry. Biosynthesis of queuosine will be studied in E. coli using 13C-labelled precursors.